Sahih Bukhari Volume 002, Book 017, Hadith Number 141.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 17. Invoking Allah for Rain (Istisqaa)
Narated By Anas bin Malik : The Prophet never raised his hands for any invocation except for that of Istisqa' and he used to raise them so much that the whiteness of his armpits became visible. (Note It may be that Anas did not see the Prophet raising his hands, but it is narrated that the Prophet used to raise his hands for invocations other than Istisqa. See Hadith No. 807 & 808 and also see Hadith No. 612, Vol. 5).